The subject of this study is to study the instabilities that have developed in the permanent east and west slopes of the Kışlaköy open pit of the Elektrik Üretim A.Ş. Afşin-Elbistan Linyitleri İşletmesi and to work out the sliding mechanism causing it. In the Kışlaköy open pit to establish if mass movement is continuing and if so which direction it would move, amount of mass to be involved and the failing model. Six movement control observations stations were established along 3 lines in each of batı (west) and doğu (east) slopes. No movement was observed in the batı (west) slopes. On the other hand up to 90 cm cumulative movement was observed at the at D24 and D18 observation station on the east (doğu) slopes. Measurements taken showed that there were failings in the lignite horizon fitting to the ‘block sliding model’. During the field study circular sliding model type failings were observed in the basal units. In the open pit in general before and after the sliding, plans were prepared of the instabilities to enable geometries could be worked out. By using these plans cross sections were prepared to show the geometry of the instabilities before it actually took place and retrospective analyses of the failing model have been carried out. Correlations have been made of the results of the retrospective analyses and resistance parameters obtained from the laboratories. It was concluded that during the movement of the mass, shear strength of the sliding planes were then represented by the shear resistance parameters. During the study it was found out that the black coloured clay band with high plasticity within the lignite horizon is one of the most crucial factors controlling slope stabilities